The Telecommunications Standards Section of the International Telecommunication Union (ITU-T) develops recommendations to facilitate the interoperation of telecommunication networks. Two of these recommendations are designated as G.992.1 (sometimes referred to as G.dmt) and G.992.2 (sometimes referred to as G.lite). Recommendation G.992.1 refers to an asymmetric digital subscriber line (ADSL) transceiver that is an ADSL industry standard for typical network access at data rates up to 8.192 Mbit/s downstream and 640 kbit/s upstream. Recommendation G.992.2, on the other hand, refers to an ADSL transceiver that is a lower data rate version of a G.992.1 ADSL transceiver. Data rates up to 1.5 Mbit/s in the downstream direction and 512 kbit/s upstream are typical with this standard. Factors such as the electrical characteristics of the customer's equipment, the distance between the subscriber and central office, and the error rate allowed all contribute to the data rate of G992.1 and G992.2 transceivers.
G.992.1 and G.992.2 transceivers both employ discrete multi-tone (DMT) technology. DMT is a form of multicarrier modulation in which the spectrum of the input signal is spread over numerous bands, also referred to as sub-channels. Each sub-channel is modulated to some carrier frequency. By working with a large number of carriers rather than a single carrier, the available channel capacity is maximized thereby optimizing performance of the transmission. A DMT symbol is the basic unit of information transmitted by an ADSL transceiver. The number of bits encoded into each sub-channel in a DMT symbol is limited by the quality of the communication channel. The quality of the communication channel can be represented by its signal-to-noise ratio (SNR). Once a connection between the central office side and the customer side has been established, the transceivers continue to monitor the changing SNR on the line and swap bits from one sub-channel to another to maintain system performance. One of the major differences between the standards is that G.992.1 downstream communication uses up to 255 sub-channels, while G.992.2 downstream communication uses up to 127 sub-channels. Thus, standard G.992.2 has a smaller available downstream bandwidth than standard G.922.1.
Both G.992.1 and G.992.2 further describe a DMT ADSL system framing standard that is designed to provide a fixed overhead per DMT symbol. A number of issues are associated with this framing approach. For example, DMT ADSL systems generally operate at different data rates depending on the associated copper loop. As such, the frame efficiency varies from one loop to the next. Generally, the frame efficiency can be determined by:
 ((line rate(kbps)−overhead(kbps))/line rate(kbps))*100%
where overhead is the Reed-Solomon (RS) codeword overhead and the sync byte. In general, the minimum fixed overhead takes up substantial bandwidth. This can be a particular problem for loops having a low total bit capacity (e.g., long loops and/or a large amount of crosstalk).
In addition, a change in the channel noise profile may require a data rate reduction to maintain the desired service margin. Likewise, spare capacity in one latency path could be beneficially used by another latency path thereby improving the overall data carrying capacity of the latency paths. In current standards, however, there is no seamless way to change the data rate (while keeping the overhead capacity the same) once a link is established between a transceiver pair without interrupting the connection, or losing a significant amount of data.
Existing techniques fail to provide a complete solution to these problems. For example, one approach allows the framing efficiency to increase for low data rate loops. However, this approach does not allow for seamless rate changes. In another approach, seamless rate changes are possible. However, if the frame efficiency is increased for the long loops, the framing efficiency on the short loops is decreased. Moreover, the overhead capacity becomes a function of the connected line rate and thus does not remain fixed and predictable.
What is needed, therefore, is a framing technique that allows for seamless rate adaptation, fixed-overhead capacity for all line rates, and high efficiency on loops that allow only low line rates.